Colour electrophotographic method in which the recording sheet is charged to its saturation voltage

ABSTRACT

An electrophotographic method for reproducing a colour picture on a recording paper sheet of an original picture by repeating an image-forming process a plurality of times in which in imparting an electric charge to the entire surface of the recording paper sheet in each image-forming process the quantity of the electric charge imparted to the recording paper sheet in an image-forming process is made higher than that in the preceding image-forming process to bring the charged voltage of the recording paper sheet around its saturated value.

United States Patent 1191 Anzai Dec. 23, 1975 COLOUR ELECTROPHOTOGRAPHICMETHOD IN WHICH THE RECORDING SHEET IS CHARGED TO ITS SATURATION VOLTAGE[75] Inventor: Masayasu Anzai, Hitachi, Japan [73] Assignee: Hitachi,Ltd., Japan [22] Filed: Feb. 1, 1974 [21] Appl. No.: 438,610

[30] Foreign Application Priority Data Feb. 5, 1973 Japan 48-13742 521US. (:1. 96/1.2; 96/1 LY 51 Int. (:1. G03G 13/01 [58] Field of Search96/12, 1 LY [56] References Cited UNITED STATES PATENTS 3,729,311 4/1973Langdon 96/1.2 3,806,340 4/1974 Sato et al. 96/l.2

OTHER PUBLICATIONS Electrofax Direct Electrophotographic Printing onCHARGED VOLTAGE Paper, RCA Review, Young et al., Dec., 1954, pp.469-484.

Characteristics of Electrophotographic Papers-Part l, Jour. Photo.Science, Axford et al., Vol. 12, 1964, pp. 20-27.

Primary Examiner-David Klein Assistant ExaminerJuds0n R. HightowerAttorney, Agent, or FirmCraig & Antonelli [57] ABSTRACT Anelectrophotographic method for reproducing a colour picture on arecording paper sheet of an original picture by repeating animage-forming process a plurality of times in which in imparting anelectric charge to the entire surface of the recording paper sheet ineach image-forming process the quantity of the electric charge impartedto the recording paper sheet in an image-forming process is made higherthan that in the preceding image-forming process to bring the chargedvoltage of the recording paper sheet around its saturated value.

25 Claims, 5 Drawing Figures V2 V3 v4 VOLTAGE US. Patent Dec. 23, 1975Sheet10f3 3,928,033

F l G.

U.S. Patent Dec. 23, 1975 Sheet20f3 3,928,033

ll mQSUO QMQWEIU VOLTAGE US. Patent Dec. 23, 1975 Sheet 3 of 3 3,928,033

CHARGED VOLTAGE HIGH VOLTAGE DIRECT CURRENT SOURCE COLOURELECTROPHOTOGRAPHIC METHOD IN WHICH THE RECORDING SHEET IS CHARGED TOITS SATURATION VOLTAGE The present invention relates to a colourelectrophotographic method, and more particularly to an improvement in amethod of imparting an electric charge to an electrophotographicsensitive paper sheet (hereinafter referred simply to as sensitive paperor recording paper) in a colour electrophotographic method.

In colour electrophotography, a reproduced colour picture is obtained bysuperimposing images of, for example, three colours, yellow, magenta andcyan, or four colours, yellow, magenta, cyan and black of an originalcolour picture on a sensitive paper. In this method, for developing acharged latent image on a sensitive paper either a powder developer or aliquid developer is utilized. Generally, the latter developer provides abetter reproduced picture. The colour electrophotography utilizing aliquid developer (wet type electrophotography) is carried out asfollows:

First, the entire surface of a sensitive paper is charged uniformlythrough an electric charge generator or an electrostatic generator, andthen the charged sensitive paper is exposed to the original picturethrough a first colour separation filter, for example a blue filter.Thus, a charged latent image corresponding to the first colourseparation filter is formed on the sensitive paper. In other words, onlythe charge corresponding to the yellow part of the original picturewhich is the complementary colour of the first colour separation filterremains on the sensitive paper proportionally to the gradation or toneof the original picture. Then, the sensitive paper carrying the chargedlatent image is immersed in a first developer solution of thecomplementary colour, yellow, of the first colour separation filter toallow the colour toner to adhere the charged latent image part of thesensitive paper. Next,

the colour toner is fixed to the sensitive paper to com plete a firstimage formation process. To provide a highly reproduced colour pictureof the original picture the above image-forming process is repeated anumber of times of the necessary number of colours while replacing thecolour separation filters and the developers to superimpose colourimages on each other on the sensitive paper.

In the above image-forming process a fixing process is performed aftereach developing process by passing the sensitive paper in a rinsingliquid and a drying liquid and by drying the sensitive paper, and thenthe next image-forming process is performed. However, since such afixing process requires a long time for providing a reproduced colourpicture, it is widely practised that a non-volatile liquid developerhaving a high viscosity of 2 to 3 cm stokes is utilized to simplify thefixing process, and the next colour image-forming process is started inthe state that a layer of the liquid developer remains on the sensitivepaper.

On the other hand, as stated above, a uniform charge impartation by thecharge generator to the entire surface of the sensitive paper isperformed as the first step of each image-forming process. The quantityof the charge to be imparted to the sensitive paper by the image-formingprocess is quite different from that in the preceding image-formingprocess. In particular, the

difference between those in the first image-forming process and thesecond and the subsequent imageforming processes is larger. On thesurface of the sensitive paper subjected to the second or the subsequentimage-forming process a liquid layer of the dispersing agent(electrically insulating liquid) containing the developer supplied inthe developing step in the preceding image-forming process remains.

As described above, the quantity of the electric charge imparted to thesensitive paper is maintained substantially constant. Consequently, thesensitive paper is subjected to the exposure step in an insufficientlycharged state in one image-forming process, while in anotherimage-forming process the sensitive paper is subjected to the exposurestep in an overcharged state. As a result, the image formation is unevenfrom process to process, so that there is the disadvantage that thegradation or tone reproduction and the colour balance of the reproducedcolour picture are poor.

Since the sensitive paper generally consists of a base paper sheet andan overlying zinc oxide (ZnO) film to be endowed with anelectrophotographic sensitive characteristic, the sensitive paper has asemiconductor characteristic. Consequently, if the sensitive paper isover-charged, the decay or extinction characteristic ,of the chargebecomes marked, so that a great difference occurs locally in the chargedvoltage on the sensitive paper depending on the local time intervalsbetween the charging and the exposure, resulting in the disadvantagethat unevenness occurs in a reproduced colour image.

Further, if the over-charging is too excessive, the sensitive paperpartly breaks down to produce specks in a reproduced colour image.

An object of the present invention is to provide an electrophotographicmethod of reproducing a colour picture from an original picture with ahigh reproducibility.

Another object of the present invention is to provide anelectrophotographic method of reproducing a colour picture improved inthe uniformity of image formation in a plurality of image formingprocesses and improved in colour balance.

Another object of the present invention is to provide anelectrophotographic method of reproducing a colour picture with highlyfaithfully reproduced gradation or tone reproduction.

A further object of the present invention is to provide anelectrophotographic method of reproducing a colour picture with a lowdegree of colour unevenness.

A still further object of the present invention is to provide anelectrophotographic method of reproducing a colour picture with nospecks resulting from the overcharging of the recording or sensitivepaper.

An electrophotographic method of reproducing a colour picture of thepresent invention, which is performed by successively superimposingcolour images on a recording paper sheet formed through pluralimage-forming processes controls the quantity of imparted electriccharges over an entire surface of the recording paper sheet in anelectric charge imparting step in each image-forming process inaccordance with the electric charging characteristic or property of-therecording paper sheet in each image-forming process.

3 Further, an electrophotographic method of reproducing a colour pictureof the present invention, which is performed by successivelysuperimposing colour images on a recording paper sheet formed throughplural image-forming processes, controls charged volt-' age of therecording paper sheet over an entire surface of the recording papersheet in an electric charge imparting step in each image forming processso that the charged voltage is rendered around its saturation value inthe condition of the recording paper sheet in each image-formingprocess.

In the above statement and hereinafter the term around the saturationvalue" refers to the saturated point and points slightly over it.

The above and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention made by referring to the preferredembodiments when taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a schematic representation of the colour electrophotographicapparatus for explaining the colour electrophotographic method accordingto the present invention;

FIG. 2a is quantitative charged voltage vs. applied voltagecharacteristics of a sensitive paper when a dispersing agent (includingno colour toner components) of a bulk resistance of IO ohms or more isused in a developer in the colour electrophotographic method accordingto the present invention and here distance and relative moving speedbetween a charge generator and a sensitive paper are kept constant;

FIG. 2b is characteristics similar to those in FIG. 20 when a dispersingagent (excluding colour toner components) of a bulk resistance of belowohms is used in a developer in the colour electrophotographic methodaccording to the present invention;

FIG. 3 is characteristics similar to that of FIG. 2a except that theabscissa represents the relative travelling speed between the chargegenerator and the sensitive paper instead of the voltage applied to thecharge generator in FIG. 2a and here distance between a charge generatorand a sensitivepaper and quantity of electric charge generated from acharge generator per unit time are kept constant; and

FIG. 4 is a circuit diagram of a charge generator used in the colourelectrophotographic method according to the present invention.

Referring to FIG. 1, a sensitive paper 12 wound around a drum 10 rotatesalong with the drum 10 at a constant speed by the signal from acontroller during which the entire surface of the sensitive paper 12 ischarged uniformly by being supplied by an electric charge generator orcorona generator with electrostatic charges. The charge generator 30consists of a core 32 connected to a high DC voltage source 100 and agrounded shield 34.

The colour image of an original picuture 52 placed on a transparentsupport 50 is projected on the uniformly charged sensitive paper 12through an exposing device 60. The rotation of the drum 10 and thetravel of the transparent support50 in the direction of the arrow aresynchronized by the controller 20. The transparent support 50 is movedby a motor 54 based on the synchronizing signal from the controller 20.

The exposing device 60 consists of a light source 62 for illuminatingthe original picture 52, a lens 64, a reflector 66 for reflecting areflected light image from the original picture 52 to the lens 64, and acolour separating filter device 68 for colour-separating the light fromthe lens 64.

The colour separating filter device 68 consists of three kinds of colourseprating filters of, for example,

blue, green and red, or four kinds of colour separating filters of, forexample, blue, green, red and white. The colour sepration filters aresuccessively charged by the controller 20 for image-forming processes.

When, in the first image-forming process, a colour sepration filter ofblue is positioned on the optical path of the reflected light from theoriginal picture 52, the charges remain only on the part of theuniformly charged sensitive paper 12 corresponding to the yellow part ofthe original picture 52 which is in complementary relationship with theblue of the filter correspondingly, to the gradation of the yellow partof the original picture 52 to form a first charge latent image on thesensitive paper 12.

A developing device 40 is formed of afirst developer reservoir 41 filledwith a developer for developing yellow which is complementary to theblue of the first colour separation filter on the sensitive paper 12, asecond developer reservoir 42 filled with a developer for developingmagenta which is complementary to green of a second colour separationfilter on the sensitive paper 12, a third developer reservoir 43 filledwith a developer for developing cyan which is complementary to red of athird colour separation filter on the sensitive paper 12, and a fourthdeveloper reservoir 44 filled with a developer for developing blackwhich is complementary to white of a fourth colour separation filter onthe sensitive paper 12. In the developer reservoirs 41 to 44 areprovided rollers 45 to 48, respectively, for well immersing the latentimage carrying sensitive paper 12 in the developers. The change in theposition of the developing device 40 relative to the drum 10 is effectedby a motor 49 based on the signal from the controller 20. The positionof the developing device 40 shown in FIG. 1 is for the state that thedeveloping step in the second image-forming process has been completed.In other words, the state that the magenta image based on the latentimage on the sensitive paper 12 corresponding to the magenta part of theoriginal picture 52 which is complementary to the green of the secondcolour separation filter is superimposed on the yellow image formed inthe first imageforming process.

Since the insulating liquid contained in the developer is carried on thesurface of the sensitive paper 12 subjected to the developing step ineach image-forming process, the carried insulating liquid is squeezedout and simultaneously therewith a colour toner is fixed to thesensitive paper 12 by a squeezing roller (fixing device) 70. Dirt orfouling on the squeezing roller is removed by a cleaning pad 72. Thenthe sensitive paper 12 is returned to the initial position to start thenext image-forming process.

The high DC voltage source in FIG. 1 is shown in a practical form. Theprimary winding 101 of a step-up I transformer 106 is connected with anAC source 102 through a inulti-contact switch 80, while the secondarywinding 103 thereof is connected with a series circuit of a capacitor104 and a rectifier 105. The supply of a high DC voltage to the core 32of the electric charge generator 30 is made from the junction point 107between the capacitor 104 and the rectifier 105.

Curves I in FIGS. 2a and 2b are charged characteristics of the sensitivepaper in the first image-forming process but not yet subjected to thedeveloping step. Since the sensitive paper is not yet exposed to thedeveloper, it is saturated in the charged voltage with a low voltageapplied to the electric charge generator. Consequently, in the firstimage-forming process the movable contact 85 of the switch 80 isconnected to the first fixed contact 81 by a signal from the controller20 to apply a DC voltage V or V between the core 32 and the shield 34 ofthe charge generator 30 for imparting charges uniformly to the sensitivepaper 12. Points (a) and (e) on the curves I in FIGS. 2a and 2b indicatethe charged voltages of the sensitive paper 12 corresponding to theapplied voltages V and V respectively, and defined as points slightlyexceeding the saturated charged voltages of the sensitive paper 12 inthe first image-forming process.

Curves n, m and IV in FIGS. 2a and 2b represent charged characteristicsof the sensitive paper 12 in the second, third and fourth image-formingprocesses, respectively.

The voltage to be applied to the charge generator 30 is set at V or V bychanging the movable contact 85 of the switch 80 to the second fixedcontact 82 by a signal from the controller 20 in the secondimage-forming process, at V or V by changing the movable contact 85 ofthe switch 80 to the third fixed contact 83 in the third image-formingprocess, and at V., or V by changing the movable contact 85 of theswitch 80 to the fourth fixed contact 84 in the fourth image-formingprocess depending on the property of the dispersing agent in thedeveloper. In this manner, the quantity of the charge imparted to thesensitive paper 12 per unit time is controlled to set the chargedvoltage of the sensitive paper 12 around the saturated value of thecharged voltage in each image-forming process. Similarly in the firstimage-forming process, points (b), (f), (c), (g), (d) and (h) on thecurves II, III, and IV in FIGS. 2a and 2b corresponding to the appliedvoltages V V V V V and V,,, respectively, are set at points slightlyover the saturated charged voltages of the sensitive paper 12 inindividual charge-forming processes.

An example of the voltages V V V and V to be applied to the chargegenerator 30 is 5.5 KV, 5.8 KV, 5.9 KV and 6.0 KV, respectively. In thiscase, the distance between the charge generator 30 and the sensitivepaper 12 is 15 mm, and the relative travelling speed therebetween is 50mm/sec.

As can be seen from the curves II, III and IV in FIGS. 2a and 2b, thecharged characteristics of the sensitive paper 12 in the second andsubsequent image-forming processes are similar to one another.Consequently, the present invention can even be achieved by only atwostep change-over of the quantity of the imparted charge per unit timeinstead of the four-step change-over described above. That is, it ispossible that the voltage applied to the charge generator is taken to beV or V in the first image-forming process, and the voltages are taken tobe V or V in the second and subsequent image-forming processes.

As another expedient for controlling the quantity of the charge to beimparted to the sensitive paper 12 per unit time the distance betweenthe charge generator'30 and the sensitive paper 12 may be adjusted ineach image-forming process.

FIG. 3 shows charged characteristics of the sensitive paper 12 when thetravelling speed of the sensitive paper 12 relative to the electriccharge generator 30 is varied. Curves I, II, III and IV are for thecharged characteristics of the sensitive paper 12 in the first, second,third and fourth image-forming processes, respectively. By setting thespeed of the drum 10 at S S S and S in the first, second, third andfourth image-forming processes, respectively, by the controller 20, thecharged voltage of the sensitive paper 12 is set at the points (i), (j),(k) and (l) for individual image-forming processes which are slightlyover the saturated charged voltages of the sensitive paper 12 in thoseprocesses. Thus, the absolute quantity of the charge imparted to thesensitive paper 12 is properly controlled depending on the state of thesensitive paper 12 in each imageforming process. Similarly to what isdescribed above, even a two-step speed change-over from S to S, canachieve the present invention in place of the four-step speedchange-over.

FIG. 4 is another example of the electric charge generator 30. Thoseparts of the charge generator 30 in FIG. 4 which perform similarfunctions to those in FIG. 1 are designated by the same referencenumerals. The core 32 of the charge generator 30 is connected to a highDC voltage source 110 similarly to the system in FIG. 1. The chargegenerator 30 is provided with a grid 36 and a resistor group connectedin series therewith consisting of first, second, third and fourthresistors 91, 92, 93 and 94. The resistances R R R and R of theseresistors are in a relation R R R R In the first image-forming process afirst resistor 91 is grounded through a switch 95. Similarly, in thesecond, third and fourth image-forming processes second, third andfourth resistors 92, 93 and 94 are grounded, respectively, through theswitch 95. The changeover of the switch 95 is performed by the signalfrom the controller 20. Thus, the charge flowing through the grid 36 iscontrolled. In other words, the quantity of the charge flowing into theground through the grid 36 in the first image-forming process ishighest, and successively decreases in the second, third and fourthimage-forming processes. In this manner, the quantity of the chargeimparted to the sensitive paper 12 per unit time in each image-formingprocess is properly controlled.

in the fourth image-forming process the switch 95 may be in an openstate instead of a closed state.

For a similar reason to those described above, the control of thequantity of the charge imparted to the sensitive paper per unit time canbe made by a two-step change-over instead of the four-step change-over.

Though in the above embodiments the original picture is moved relativeto the exposing device and the sensitive paper is moved relative to thecharge generator, the exposing device and the charge generator may bemoved contrary thereto.

I claim:

1. An electrophotographic method of reproducing a colour picture on arecording paper sheet from an original picture while successivelychanging a plurality of colour filters corresponding to the number ofrequired colours and the same number of developers individuallyproducing the complementary colours of the corresponding colour filtersand repeating a number of times corresponding to the number of thefilters the image-forming process comprising the steps of imparting anelectric charge to the entire surface of the recording paper sheet,forming a first charge latent image on the recording paper sheetcorresponding to the complementary colour of the first colour filter ofthe original picture through the first colour filter, developing a firstcolour image on the recording paper sheet on which the first latentimage has been formed by a first developer producing the complementarycolour of the first colour filter, and fixing the first colour image onthe recording paper sheet, characterized in that in the electric chargeimparting step in each imageforming process the charged voltage of therecording paper sheet is selected around its saturation value.

2. A colour electrophotographic method according to claim 1, whereinquantity of the electric charge imparted to the recording paper sheetper unit time in each of second and the subsequent image-formingprocesses is higher than that in the first image-forming process.

3. A colour electrophotographic method according to claim 2, whereinquantity of the electric charge imparted to the recording paper sheetper unit time in at least one image-forming process is higher than thatin the preceding imaage-forming process.

4. A colour electrophotographic method according to claim 1, whereinquantity of the electric charge imparted to the recording paper sheet ineach of the second and the subsequent image-forming processes is higherthan that in the first image-forming process.

5. A colour electrophotographic method according to claim 4, whereinquantity of the electric charge imparted to the recording paper sheet inat least one image-forming process is higher than that in the precedingimage-forming process.

6. A colour electrophotographic method according to claim 2, whereincharge impartation to the recording paper sheet in each image-formingprocess is made through electric charge generator means and the controlof quantity of the electric charge imparted to the recording paper sheetper unit time is made by controlling the voltage applied to the electriccharge generating means.

7. A colour electrophotographic method according to claim 2, whereincharge impartation to the recording paper sheet in each image-formingprocess is made through electric charge generating means and the controlof quantity of the electric charge imparted to the recording paper sheetper unit time is made by controlling the distance between the electriccharge generating means and the recording paper sheet.

8. A colour electrophotographic method according to claim 2, whereincharge impartation to the recording paper sheet in each image-formingprocess is made through electric charge generating means and control ofthe quantity of the electric charge imparted to the recording papersheet per unit time is made by controlling the grid resistance of theelectric charge generating means.

9. A colour electrophotographic method according to claim 3, whereincharge impartation to the recording paper sheet in each image-formingprocess is made through electric charge generating means and the control of quantity of the electric charge imparted to the recording papersheet per unit time is made by controlling the voltage applied to theelectric charge generating means.

10. A colour electrophotographic method according to claim 3, whereincharge impartation to the recording paper sheet in each image-formingprocess is made through electric charge generating means and the controlof quantity of the electric charge imparted to the recording paper sheetper unit time is made by controlling the distance between the electriccharge generat ing means and the recording paper sheet.

11. A colour electrophotographic method according to claim 3, whereincharge impartation to the recording paper sheet in each image-formingprocess is made through electric charge generating means and the controlof quantity of the electric charge imparted to the recording paper sheetper unit time is made by controlling the grid resistance of the electriccharge generating means.

12. A colour electrophotographic method according to claim 4, whereincharge impartation to the recording paper sheet in each image-formingprocess is made through electric charge generating means and the controlof quantity of the electric charge imparted to the recording paper sheetis made by controlling the time period during which the electric chargeis imparted by the electric charge generating means to the recordingpaper sheet.

13. A colour electrophotographic method according to claim 5, whereincharge impartation to the recording paper sheet in each image-formingprocess is made through electric charge generating means and the controlof quantity of the electric charge imparted to the recording paper sheetis made by controlling the time period during which the electric chargeis imparted by the electric charge generating means to the recordingpaper sheet.

14. In an electrophotographic method for reproducing a color picture ona recording sheet from an original picture in which a plurality of colorimages are formed in superposed relationship on said recording sheet,each color image being formed by an imageforming process comprisingimparting an electric charge to the entire surface of said recordingsheet, forming a charge latent image on said recording sheetcorresponding to the respective color image to be formed on saidrecording sheet, developing the respective color image on the recordingsheet on which the latent image has been formed by a liquid developercontaining a developing agent and an electrically insulating liquid, andfixing the respective color image on the recording sheet, the recordingsheet when subjected to second and subsequent image-forming processesbeing wet with said electrically insulating liquid, the improvementwherein sufficient electric charge is imparted to said recording sheetduring each respective image-forming process so that the charged,voltage of the recording sheet in each respective image-forming processis selected at or slightly above its saturation value.

15. A color electrophotographic method according to claim 14, whereinthe value of the saturation charged voltage of said recording sheet inthe first image-forming process is different from the values of thesaturation charged voltages of the recording sheet in second andsubsequent image-forming processes.

16. A color electrophotographic method according to claim 15, whereinthe values of the saturated charged voltages of the recording sheet inrespective image-forming processes are different from one another.

17. A color electrophotographic method according to claim 16, whereincharge impartation to the recording sheet in each image-forming processis made through electric charge generator means and control of thequantity of the electric charge imparted to the recording sheet per unittime is made by controlling the voltage applied to the electric chargegenerating means.

18. A color electrophotographic method according to claim 16, whereincharge impartation to the recording sheet in each image-forming processis made through electric charge generating means and control of thequantity of the electric charge imparted to the recording sheet per unittime is made by controlling the distance between the electric chargegenerating means and the recording sheet.

19. A color electrophotographic method according to claim 16, whereincharge impartation to the recording sheet in each image-forming processis made through electric charge generating means and control of thequantity of the electric charge imparted to the recording sheet per unittime is made by controlling the grid resistance of the electric chargegenerating means.

20. A color electrophotographic method according to claim 16, whereinsaid liquid developer is applied to said recording sheet by means of aroller.

21. A color electrophotographic method according to claim 15, whereincharge impartation to the recording sheet in each image-forming processis made through electric charge generator means and control of thequantity of the electric charge imparted to the recording sheet per unittime is made by controlling the voltage applied to the electric chargegenerating means.

22. A color electrophotographic method according to claim 15, whereincharge impartation to the recording sheet in each image-forming processis made through electric charge generating means and control of thequantity of the electric charge imparted to the recording sheet per unittime is made by controlling the distance between the electric chargegenerating means and the recording sheet.

23. A color electrophotographic method according to claim 15, whereincharge impartation to the recording sheet in each image-forming processis made through electric charge generating means and control of thequantity of the electric charge imparted to the recording sheet per unittime is made by controlling the grid resistance of the electric chargegenerating means.

24. A color electrophotographic method according to claim 15, whereinsaid liquid developer is applied to said recording sheet by means of aroller.

25. A color electrophotographic method according to claim 14, whereinsaid liquid developer is applied to said recording sheet by means of aroller.

1. AN ELECTROPHOTOGRAPHIC METHOD OF REPRODUCING A COLOUR PICTURE ON ARECORDING PAPER SHEET FROM AN ORIGINAL PICTURE WHILE SUCESSIVELYCHANGING A PLURALITY OF COLOUR FILTERS CORRESPONDING TO THE NUMBER OFREQUIRED COLOURS AND THE SAME NUMBER OF DEVELOPERS INDIVIDUALLYPRODUCING THE COMPLEMENTARY COLOURS OF THE CORRESPONDING COLOUR FILTERSAND REPEATING A NUMBER OF TIMES CORRESPONDING THE NUMBER OF THE FILTERSTHE IMAGE-FORMING PROCESS COMPRISING THE STEPS OF IMPARTING AN ELECTRICCHARGE TO THE ENTIRE SURFACE OF THE RECORDING PAPER SHEET, FORMING AFIRST CHARGE LATENT IMAGE ON THE RECORDING PAPER SHEET CORRESPONDING TOTHE COMPLEMENTARY COLOUR OF THE FIRST COLOUR FILTER OF THE ORIGINALPICTURE THROUGH THE FIRST COLOUR FILTER, DEVELOPING A FIRST COLOUR IMAGEON THE RECORDING PAPER SHEET ON WHICH THE FIRST LATENT IMAGE HAS BEENFORMED BY A FIRST DEVELOPER PRODUCING THE COMPLEMENTARY COLOUR OF THEFIRST COLOUR FILTER, AND FIXING THE FIRST COLOUR IMAGE ON THE RECORDINGPAPER SHEET, CHARACTERIZED IN THAT IN THE ELECTRIC
 2. A colourelectrophotographic method according to claim 1, wherein quantity of theelectric charge imparted to the recording paper sheet per unit time ineach of second and the subsequent image-forming processes is higher thanthat in the first image-forming process.
 3. A colour electrophotographicmethod according to claim 2, wherein quantity of the electric chargeimparted to the recording paper sheet per unit time in at least oneimage-forming process is higher than that in the precedingimaage-forming process.
 4. A colour electrophotographic method accordingto claim 1, wherein quantity of the electric charge imparted to therecording paper sheet in each of the second and the subsequentimage-forming processes is higher than that in the first image-formingprocess.
 5. A colour electrophotographic method according to claim 4,wherein quantity of the electric charge imparted to the recording papersheet in at leaSt one image-forming process is higher than that in thepreceding image-forming process.
 6. A colour electrophotographic methodaccording to claim 2, wherein charge impartation to the recording papersheet in each image-forming process is made through electric chargegenerator means and the control of quantity of the electric chargeimparted to the recording paper sheet per unit time is made bycontrolling the voltage applied to the electric charge generating means.7. A colour electrophotographic method according to claim 2, whereincharge impartation to the recording paper sheet in each image-formingprocess is made through electric charge generating means and the controlof quantity of the electric charge imparted to the recording paper sheetper unit time is made by controlling the distance between the electriccharge generating means and the recording paper sheet.
 8. A colourelectrophotographic method according to claim 2, wherein chargeimpartation to the recording paper sheet in each image-forming processis made through electric charge generating means and control of thequantity of the electric charge imparted to the recording paper sheetper unit time is made by controlling the grid resistance of the electriccharge generating means.
 9. A colour electrophotographic methodaccording to claim 3, wherein charge impartation to the recording papersheet in each image-forming process is made through electric chargegenerating means and the control of quantity of the electric chargeimparted to the recording paper sheet per unit time is made bycontrolling the voltage applied to the electric charge generating means.10. A colour electrophotographic method according to claim 3, whereincharge impartation to the recording paper sheet in each image-formingprocess is made through electric charge generating means and the controlof quantity of the electric charge imparted to the recording paper sheetper unit time is made by controlling the distance between the electriccharge generating means and the recording paper sheet.
 11. A colourelectrophotographic method according to claim 3, wherein chargeimpartation to the recording paper sheet in each image-forming processis made through electric charge generating means and the control ofquantity of the electric charge imparted to the recording paper sheetper unit time is made by controlling the grid resistance of the electriccharge generating means.
 12. A colour electrophotographic methodaccording to claim 4, wherein charge impartation to the recording papersheet in each image-forming process is made through electric chargegenerating means and the control of quantity of the electric chargeimparted to the recording paper sheet is made by controlling the timeperiod during which the electric charge is imparted by the electriccharge generating means to the recording paper sheet.
 13. A colourelectrophotographic method according to claim 5, wherein chargeimpartation to the recording paper sheet in each image-forming processis made through electric charge generating means and the control ofquantity of the electric charge imparted to the recording paper sheet ismade by controlling the time period during which the electric charge isimparted by the electric charge generating means to the recording papersheet.
 14. In an electrophotographic method for reproducing a colorpicture on a recording sheet from an original picture in which aplurality of color images are formed in superposed relationship on saidrecording sheet, each color image being formed by an image-formingprocess comprising imparting an electric charge to the entire surface ofsaid recording sheet, forming a charge latent image on said recordingsheet corresponding to the respective color image to be formed on saidrecording sheet, developing the respective color image on the recordingsheet on which the latent image has been formed by a liquid developercontaining a developing agent and an electrically insulating liquid, andfixiNg the respective color image on the recording sheet, the recordingsheet when subjected to second and subsequent image-forming processesbeing wet with said electrically insulating liquid, the improvementwherein sufficient electric charge is imparted to said recording sheetduring each respective image-forming process so that the charged voltageof the recording sheet in each respective image-forming process isselected at or slightly above its saturation value.
 15. A colorelectrophotographic method according to claim 14, wherein the value ofthe saturation charged voltage of said recording sheet in the firstimage-forming process is different from the values of the saturationcharged voltages of the recording sheet in second and subsequentimage-forming processes.
 16. A color electrophotographic methodaccording to claim 15, wherein the values of the saturated chargedvoltages of the recording sheet in respective image-forming processesare different from one another.
 17. A color electrophotographic methodaccording to claim 16, wherein charge impartation to the recording sheetin each image-forming process is made through electric charge generatormeans and control of the quantity of the electric charge imparted to therecording sheet per unit time is made by controlling the voltage appliedto the electric charge generating means.
 18. A color electrophotographicmethod according to claim 16, wherein charge impartation to therecording sheet in each image-forming process is made through electriccharge generating means and control of the quantity of the electriccharge imparted to the recording sheet per unit time is made bycontrolling the distance between the electric charge generating meansand the recording sheet.
 19. A color electrophotographic methodaccording to claim 16, wherein charge impartation to the recording sheetin each image-forming process is made through electric charge generatingmeans and control of the quantity of the electric charge imparted to therecording sheet per unit time is made by controlling the grid resistanceof the electric charge generating means.
 20. A color electrophotographicmethod according to claim 16, wherein said liquid developer is appliedto said recording sheet by means of a roller.
 21. A colorelectrophotographic method according to claim 15, wherein chargeimpartation to the recording sheet in each image-forming process is madethrough electric charge generator means and control of the quantity ofthe electric charge imparted to the recording sheet per unit time ismade by controlling the voltage applied to the electric chargegenerating means.
 22. A color electrophotographic method according toclaim 15, wherein charge impartation to the recording sheet in eachimage-forming process is made through electric charge generating meansand control of the quantity of the electric charge imparted to therecording sheet per unit time is made by controlling the distancebetween the electric charge generating means and the recording sheet.23. A color electrophotographic method according to claim 15, whereincharge impartation to the recording sheet in each image-forming processis made through electric charge generating means and control of thequantity of the electric charge imparted to the recording sheet per unittime is made by controlling the grid resistance of the electric chargegenerating means.
 24. A color electrophotographic method according toclaim 15, wherein said liquid developer is applied to said recordingsheet by means of a roller.
 25. A color electrophotographic methodaccording to claim 14, wherein said liquid developer is applied to saidrecording sheet by means of a roller.